Bi-directional switches switch high currents through their conduction electrodes while blocking high voltages applied to the conduction electrodes. Bi-directional switches are used in a variety of electrical systems. A typical bi-directional switch is specified to supply high currents, which may range from several Amperes of maximum current to several hundreds of Amperes depending on the specific switch and application, while blocking relatively high voltages, e.g. of at least 25 V without breaking down.
Bi-directional switches are typically implemented using electromechanical switches or a configuration of semiconductor devices, e.g. power transistors. However, in one direction standard power transistors do not have a technically meaningful blocking voltage, making them unidirectional devices. Consequently, current bi-directional switches typically are implemented using two separate serially coupled power MOSFETs. The separate MOSFETs are formed on separate semiconductor dice, and often housed in separate packages, which results in a high manufacturing cost and a large area occupied on a circuit board. When the separate MOSFET dice are housed in a single package and interconnected with wire bonds, the area occupied on a circuit board is reduced but the manufacturing cost is still too high for many applications.
U.S. Pat. Nos. 7,282,406, 7,297,603, 7,537,970, 7,910,409, 8,101,969 and 8,530,284 all disclose an integrated circuit with several different transistors integrated on the same circuit, including a p-channel bi-directional trench power transistor for battery charging protection. The transistor comprises two vertical trenches between which a body is present. The body is separated from current carrying electrodes above and below the body by high-voltage regions with a lesser doping concentration than the electrodes. However, this bi-directional trench power transistor has an inherent parasitic bipolar transistor formed by the body and the high voltage drift regions. Furthermore, it is not suitable for operation with high voltages, such as of at least 20 or more, e.g. up to 40 V or more, and or high currents. e.g. above 1 mA, up to 1 A or more.
In the off-state and when the drain-source voltage is low, current flows between drain and body, which can increase suddenly when the power transistor is in the avalanche regime of the parasitic bipolar transistor. As a result, the potential barrier of the junction formed between the body and high voltage region below the body is lowered in the middle. This causes a current flowing to the source which results in a sudden decrease of the drain-source voltage. The focalization of the current flow in the middle can cause thermal runaway and device destruction. Furthermore, the blocking voltage of the bi-directional trench power transistor is affected when the snapback holding voltage of the parasitic bipolar transistor is less than the specified blocking voltage of the bi-directional trench power transistor.